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If a cells need for atp is significantly greater than

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If a cell’s need for ATP is significantly greater than its need for NADPH, it may bypass the production of reducing power throughcyclic photophosphorylation. Only PSI is used during cyclic photophosphorylation; the high-energy electron of the PSI reactioncenter is passed to an ETS carrier and then ultimately returns to the oxidized PSI reaction center pigment, thereby reducing it.2Exercise222+
Access for free at OpenStax8.5.4Figure: Eukaryotes and cyanobacteria carry out oxygenic photosynthesis, producing oxygen, whereas other bacteria carry outanoxygenic photosynthesis, which does not produce oxygen.
Access for free at OpenStax8.5.5Figure: (a) PSI and PSII are found on the thylakoid membrane. The high-energy electron from PSII is passed to an ETS,which generates a proton motive force for ATP synthesis by chemiosmosis, and ultimately replaces the electron lost by the PSIreaction center. The PSI reaction center electron is used to make NADPH. (b) When both ATP and NADPH are required, noncyclicphotophosphorylation (in cyanobacteria and plants) provides both. The electron flow described here is referred to as the Z-scheme
Access for free at OpenStax8.5.6(shown in yellow in [a]). When the cell’s ATP needs outweigh those for NADPH, cyanobacteria and plants will use only PSI, andits reaction center electron is passed to the ETS to generate a proton motive force used for ATP synthesis.Why would a photosynthetic bacterium have different pigments?Light-Independent ReactionsAfter the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules (havinglifespans of millionths of a second), photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which cansurvive for hundreds of millions of years, for long-term energy storage. The carbon comes from CO , the gas that is a wasteproduct of cellular respiration.The Calvin-Benson cycle (named for Melvin Calvin [1911–1997] and Andrew Benson [1917–2015]), the biochemical pathwayused for fixation of CO , is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts.The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration(seeAppendix Cfor a detailed illustration of the Calvin cycle).Fixation: The enzyme ribulose bisphosphate carboxylase (RuBisCO) catalyzes the addition of a COto ribulose bisphosphate(RuBP). This results in the production of 3-phosphoglycerate (3-PGA).Reduction: Six molecules of both ATP and NADPH (from the light-dependent reactions) are used to convert 3-PGA intoglyceraldehyde 3-phosphate (G3P). Some G3P is then used to build glucose.

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